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SQL connection (JOIN)
It is used to combine SQL join rows from two or more tables.
The following figure shows the seven kinds of usage LEFT JOIN, RIGHT JOIN, INNER JOIN , OUTER JOIN relevant.
SQL JOIN clause is used to combine two or more rows from the table, the common field between the tables based.
The most common type of JOIN: SQL INNER JOIN (simple JOIN).
SQL INNER JOIN JOIN returns all rows satisfying the conditions from the plurality of tables.
Different SQL JOIN
Different SQL JOIN type used:
the INNER JOIN: If the table has at least one match, the return line
LEFT JOIN: the right table even if there is no match, it returns all rows from the left table
RIGHT JOIN: even if there is no match left table, from right table returns all rows
FULL JOIN: As long as there is a match where a table, return line
SQL INNER JOIN keywords
INNER JOIN keyword return line is present in at least one matching table.
grammar
SELECT column_name(s)
FROM table1
INNER JOIN table2 ON table1.column_name=table2.column_name;
or:
SELECT column_name(s)
FROM table1
JOIN table2 ON table1.column_name=table2.column_name;
SQL LEFT JOIN Keyword
LEFT JOIN keyword from the left table (table1) returns all rows, even if the right table (table2) does not match. If there is no match in the right table, the result to NULL.
grammar
SELECT column_name(s)
FROM table1
LEFT JOIN table2
ON table1.column_name=table2.column_name;
or:
SELECT column_name(s)
FROM table1
LEFT OUTER JOIN table2
ON table1.column_name=table2.column_name;
Table on the left websites, the right table is access_log. Right table no data will be displayed.
LEFT JOIN keyword returns all rows from the left table, right table even if there is no match.
SQL RIGHT JOIN Keyword
RIGHT JOIN right keyword table (table2) returns all rows, even if the left table (table1) does not match. If there is no match left table, the result is NULL.
grammar
SELECT column_name(s)
FROM table1
RIGHT JOIN table2
ON table1.column_name=table2.column_name;
or:
SELECT column_name(s)
FROM table1
RIGHT OUTER JOIN table2
ON table1.column_name=table2.column_name;
access_log as the left table, websites as the right table
RIGHT JOIN keyword returns all rows from the right table, even if the table does not match left.
SQL FULL OUTER JOIN keywords
FULL OUTER JOIN keyword as long as there is a match left table (table1) and the right table (Table2) wherein a table, the row is returned.
FULL OUTER JOIN keyword combines the results of LEFT JOIN and RIGHT JOIN.
grammar
SELECT column_name(s)
FROM table1
FULL OUTER JOIN table2
ON table1.column_name=table2.column_name;
MySQL does not support FULL OUTER JOIN.
FULL OUTER JOIN keyword returns the left and right tables all rows.
If no match "websites" table row "access_log" or "access_log" table row "websites" does not match the table will list these lines.
